Whether at sea or on land, the first stages in the production of a new hydrocarbon well--an oil well--are the drilling of the well bore itself through the various formations within the earth's crust beneath the drilling rig, followed by "casing" (the introduction and cementing into position of piping which will serve to support and line the bore) and the placing in the bore, at the depth of a formation of interest, of a device known as a packer, into which inner tubing (of smaller diameter than the casing) can subsequently be lodged.
The next work carried out is normally some programme of testing, for the purpose of evaluating the production potential of the chosen formation. The testing procedure usually involves the measurement of downhole temperatures and pressures, in both static and flow conditions (the latter being when fluid from the relevant formation is allowed to flow into and up the well), and the subsequent calculation of various well parameters. To collect the necessary data there is lowered into the well a test string--a length of tubing containing the tools required for testing. The flow of fluid from the formation of interest into the well bore and thus to the test tools is controlled by a valve known as a sub-surface control valve.
The operation of the various tools include in the downhole test string can be effected using one of three main types of mechanism. These types are those actuated by reciprocal motion of the pipe string (the inner tube, of which the test string constitutes a part), by rotational motion of the pipe string, or by changes in the pressure differential between the tubing and the annular space which surrounds it in the well-- hereinafter referred to simply as "the annulus". Test strings wherein the tools thereof are actuated by changes in annulus pressure are at present much in vogue, and it is this type of actuation mechanism that is to be employed with the apparatus of the invention.
A mechanism of the annulus pressure-responsive type requires the provision and maintenance of a fixed "reference" pressure within the tool. This, used in conjunction with an adjustable (and higher) annulus pressure, allows the establishment of the chosen pressure differential necessary to control the operation of the appropriate component of the test string. The achievement of such a fixed reference pressure is the subject of our co-pending British Patent Application No. 89/07,098.1 (Publication No: 2,229,748A; FN P1049).
Following completion of the well testing procedure, it is necessary safely to "shut down" the test tools, and then to remove the test string from the packer assembly and pull it to the surface. These operations do, however, require careful control and planning. In the case of pressure-differential-actuated test tools, for example, the string will, at the end of testing, still contain the high pressure reference gas which has been used in creating the required differentials. It is extremely desirable for this gas in some way to be vented before the string reaches the well head, so that there are no potentially dangerous pressures trapped within the tools when the test string is received at the surface.
Additionally, it is an advantage if there be incorporated within the test string some means of isolating the upper portion of the tubing thereof, and of subsequently providing a route for communication between this tubing and the annulus, so that tubing-contained well liquid above the test string can then be circulated out of the tubing before it is raised to the surface. The isolation is conveniently accomplished using a ball valve suitably placed near the top of the test string, and such a ball valve particularly suitable for effecting this isolation is described in our co-pending British Patent Application No. 89/09,903.0 (Publication No: 2,231,069A; FN P1062). However, reliance upon a single valve is not advisable, and consequently there is a strong case in favour of the utilisation of a second valve in the test apparatus. This latter valve can then be used either in addition to the main valve or, in the event of the latter not operating correctly, as an alternative thereto.
The present invention seeks to facilitate the procedure for discontinuation of an oil well testing programme by providing apparatus for the venting and isolation procedures just described. Moreover, the apparatus permits those operations to be carried out as an automatic sequence, following the application of a single actuating pressure pulse to the annulus. For the venting of the reference gas, the invention suggests pressure release apparatus having two spaced pistons located at opposite ends of a chamber filled with that gas and blocking both a gas vent to annulus and a hydraulic liquid passageway (to further up the test string), the pistons being held together by a shear pin until the application of a predetermined pressure (higher than the gas reference pressure) at the outside ends of those pistons causes the pin to shear, allowing sequential movement of the two pistons towards each other, with the effect of firstly opening the gas vent to annulus, and secondly opening the passageway to a chamber of hydraulic liquid.
The hydraulic liquid pressure within this passageway then causes actuation of ball valve apparatus for isolating the upper section of tubing. This apparatus is in the form of a ball-valve-driving piston blocking another passageway for hydraulic liquid, which piston is forced to move under the influence of the pressure, breaking a restraining shear pin as it does so, and closing the ball valve while opening this other hydraulic liquid passageway, permitting transfer of hydraulic pressure to apparatus for venting the contents of the tubing to annulus. Finally, this venting apparatus contains a longitudinally-movable sleeve member the position of which determines whether or not flow is permitted, via a vent port, from the test string tubing to the annulus.